Are Mega Testers Accurate for Testing Puck Thyristors?

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Discussion Overview

The discussion revolves around the accuracy of using mega testers to evaluate puck type thyristors in a furnace application. Participants explore the implications of testing methods, clamping requirements, and the relationship between low current testing and high current performance.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the accuracy of using a mega tester with 250V and running 24Vdc through the thyristors, particularly regarding the necessity of clamping and the impact of current levels on performance.
  • Another participant asserts that if a thyristor works at 1A, it should function at its rated current, emphasizing the importance of proper heat sink maintenance and clamping pressure during replacement.
  • Several participants inquire about the necessity of clamping during testing, with one suggesting that clamping may be crucial for ensuring proper electrical connection, especially for older devices.
  • It is noted that while new devices may function without clamping, mechanical and thermal stresses could affect the performance of older devices if not clamped.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of clamping during testing, with some suggesting it is essential for proper operation, while others believe testing without clamping may be acceptable under certain conditions. The discussion remains unresolved regarding the overall accuracy of the testing method.

Contextual Notes

There are unresolved questions about the specific conditions under which clamping is necessary, the effects of mechanical and thermal stresses on older devices, and the implications of testing at lower currents compared to rated currents.

roro36
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I just started working on a furnace that uses the puck type thyristors for the main power switching. Every now and then, one of them blows and we replace it. We ususally use a mega tester to test wheteher they have blown or not i.e. put 250V across and short the gate to the anode. A zero impedance means its working fine. I am however not too sure how accurate this is for testing? In the data sheets of these thyristors it says they need to be clamped with 500kg of force in some cases. Now by running 24Vdc through these thyristors is this test being performed adequately? Does the device need to be clamped and does the amount of current running through it matter. If its rated for 1300A and I run 1A through it and t works, doees this mean it ill work for high amps or not?

Thanks
 
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roro36,

Yes. SCRs and TRIACs work or don't work, no middle ground; so if it works @ 1A, it should work @ rated current. Make sure when replacing one to clean the heat sinks & use a good quality heat sink paste. Use a torque wrench to get the clamping pressure as close as possible.

Fish
 
How important is the clamping? If testing without is fine, what is the importance of the clamping in the machine?
 
roro36 said:
How important is the clamping? If testing without is fine, what is the importance of the clamping in the machine?

Maintaining proper thermal and electrical conduction.

Fish
 
Sorry, stupid previous question, meant for testing? Is it not necessary to have the SCR or Diode clamped for testing?
 
roro36,

AFAIK, the only reason for clamping during testing is to ensure proper electrical connection. If it works "unclamped", you should be good to go, if it does not, you may want to clamp it and re-test it. These devices have a large contact area between the metal ends and the silicon that may not perform properly unless clamped. In most cases a "new device" will work just fine w/o clamping, but mechanical and thermal stresses may prevent older devices from operating properly w/o pressure.Fish
 
Thanks.